Thermal Diffusion of Gas Mixtures and Forces between Unlike Molecules

The Chapman-Enskog formulae for thermal diffusion in a mixture of gases have been utilized to interpret the data on several gas mixtures at different temperatures but having the same composition. Assuming the Lennard-Jones 12:6 interaction potential also for unlike molecules, the minimum mutual potential 12 for interaction between two unlike molecules has been calculated from the temperature dependence of the thermal diffusion ratio k T . These determinations are expected to provide the most accurate value for this interaction as thermal diffusion is far more sensitive to the laws of interaction than the other transport properties which have been hitherto used to estimate 12 . The distance r 12 at which the potential vanishes is calculated from the experimental data on the inter-diffusion coefficient D 12 by utilizing these values of 12 . Thus 12 and r 12 are determined purely from experimental data. Equations have been developed to correlate 12 and r 12 with 11 , 22 and r 11 , r 22 , and these relations have been tested with the help of the experimental values. It is found that our theoretical relations give much better agreement than the assumed geometric mean relation for 12 and the arithmetic mean relation for r 12 . Equations have also been developed for μ 12 and λ 12 and their theoretically obtained values compared with the experimental results.